1. (a) Field of the Invention
The present invention relates to a centrifugal turbo machine and more particularly to a centrifugal turbo machine which can properly control an axial thrust resulting from the difference in static pressure at the front and rear of an impeller in a centrifugal pump or compressor.
2. (b) Description of the Related Art
In general, a centrifugal turbo machine gives kinetic energy (dynamic pressure) to fluid by means of a reaction due to rotation of a rotor and converts the kinetic energy into pressure energy (static pressure). Examples of such a centrifugal turbo machine can include a centrifugal pump, a centrifugal compressor, and the like.
The structure shown in FIG. 1 was usually used in the centrifugal turbo machine for converting the kinetic energy given to the fluid into the pressure energy.
Referring to the figure, the centrifugal turbo machine 10 includes a rotation axis 11, an impeller 12, a volute casing 13, and seals 14a and 14b. 
The rotation axis 11 is rotatably coupled to the volute casing 13 through a bearing 15.
The impeller 12 is coupled to the rotation axis 11 and sucks in fluid with a centrifugal force generated by the rotation of the impeller 12.
The fluid sucked by the impeller 12 flows in the volute casing 13 and the dynamic pressure of the sucked fluid is converted-into the static pressure therein. That is, the kinetic energy of the sucked fluid is converted into the pressure energy.
The seals 14a and 14b prevent leakage of the fluid to enhance efficiency of the centrifugal turbo machine 10, and are provided at the front and rear of the impeller 12, respectively.
The operation principle of the centrifugal turbo machine 10 is now described.
In order to suck fluid into the volute casing 13, the impeller 12 is rotated in the closed volute casing 13. Then, a centrifugal force is generated in the impeller 12 and the fluid is sucked with the centrifugal force. The sucked fluid flows in the volute casing 13 and the dynamic pressure of the fluid is converted into the static pressure in the volute casing 13, thereby obtaining the pressure energy.
However, a part of the fluid sucked by the impeller 12 does not flow in the volute casing 13 but flows through gaps 16 of the seals 14a and 14b positioned at the front and rear of the impeller 12. The fluids flowing through the gaps 16 of the seals 14a and 14b at the front and rear of the impeller 12 are different in pressure from each other, thereby generating an axial thrust. That is, as shown in FIG. 1, the shapes of the front and rear surfaces of the impeller 12 are different from each other and the space between the front surface of the impeller 12 and a wall surface of the volute casing 13 adjacent thereto and the space between the rear surface of the impeller 12 and a wall surface of the volute casing 13 adjacent thereto are different in area. Accordingly, the pressure is different by places and the pressures at the outlets of the seals 14a and 14b are different from each other, so that an axial thrust is generated toward the front of the impeller 12 from the rear thereof.
Referring to FIG. 2 which illustrates an angular velocity ratio distribution of the fluid flowing through the gap 16 of the seal 14b at the rear of the impeller 12, the fluid has an angular velocity ranging about 0.6 to 0.8 before passing through the seal 14b and has an angular velocity of about 0.8 or more after passing through the seal 14b. 
Therefore, as shown in FIG. 3, the static pressure is varied in the radial direction. That is, it can be seen that the static pressure, which was about 220,000 Pa before passing through the seal 14b, is gradually decreased during passing through the seal 14b. Since the static pressure of the fluid having passed through the seal 14b is decreased toward the lower side of the seal 14b but the static pressure is not constant as a whole, the axial thrust cannot be removed. The axial thrust is delivered to the bearing 15 coupled with the rotation axis 11. As the magnitude of the axial thrust delivered to the bearing 15 is increased, the pressure applied to the bearing 15 is increased, thereby causing the damage of the bearing 15.
The damage of the bearing 15 due to the axial thrust hinders the stable operation of the centrifugal turbo machine 10. Therefore, the axial thrust should be reduced to secure the stable operation of the centrifugal turbo machine 10. The different in static pressure acting on the front and rear surfaces of the impeller 12 should be decreased to reduce the axial thrust.
Conventionally, in order to decrease the difference in static pressure acting on the front and rear surfaces of the impeller 12, the radii of the seals 14a and 14b provided at the front and rear of the impeller 12 were changed, thereby changing the space between the impeller 12 and the volute casing 13. That is, a method was used which increases the radius of the seal 14b at the rear of the impeller 12 on which the great axial thrust usually acts, thereby reducing the axial thrust at the rear of the impeller 12.
However, the reduction of the axial thrust by means of the change in radius of the seals requires much time and cost for manufacturing the centrifugal turbo machine.